Polylactide can be produced from plants and its strength and physical properties are relatively better than those of other biodegradable resins. Accordingly, polylactide has rapidly become a focus of attention as a material alternative to existing plastics or fibers that are made from petroleum.
However, the biodegradability of polylactide is lower than that of other commonly known biodegradable plastics such as polyhydroxybutyric acid, polycaprolactone, or polybutylene succinate. For example, the quantities of biodegradable plastic-degrading bacteria in these plastics can be represented by the following sequence: polyhydroxybutyric acid≧polycaprolactone>polybutylene succinate>polylactide. Unlike another commonly known biodegradable plastic, i.e., aliphatic polyester, polylactide is aliphatic polyester formed by an α-ester bond. Thus, polylactide has specific properties. For example, it is not degraded by a lipase, esterase, or polyhydroxybutyric acid-degrading enzyme.
Accordingly, polylactides, which are excellent in terms of ready biodegradability, are strongly desired particularly in markets in the fields of agriculture or civil engineering where rapidly biodegradable products such as multi-purpose films or vegetation nets are strongly desired.
Meanwhile, techniques have been proposed in the past wherein a naturally occurring polymer such as starch is blended into biodegradable aliphatic polyester in order to improve cost, formability, and biodegradability of products (e.g., JP Patent Publication (Kokai) Nos. 5-320524 A (1993) and 5-331315 A (1993)).
When these blending techniques as such are applied to polylactide, however, the good mechanical strength or physical properties thereof are lost. Thus, a polylactide resin composition useful as a material for multi-purpose films, vegetation nets, etc. could not be obtained.
An object of the present invention is to provide a novel rapidly biodegradable polylactide resin composition by overcoming drawbacks of the prior art while maintaining good mechanical strength and physical properties of polylactide.
The present inventors have conducted concentrated studies in order to attain the above object. As a result, they have found that a polylactide resin composition having excellent biodegradability can be obtained while maintaining good physical properties and strengths of polylactide with the addition of a specific protein to polylactide or a mannan-containing polylactide composition. This has led to the completion of the present invention.